1. Technical Field of the Invention
The present invention relates to a porous polymer film, a process for producing the same, and a method for using the same. Specifically, the present invention relates to a wholly new porous film which is obtained by coagulating polymer particles in an emulsion into the form of a film while substantially maintaining their particle shape and leaving the spaces between the particles as pores. The porous film of the present invention is usable for various applications, and is especially suitable for use in a recording material to be printed or recorded (hereinafter, referred to as a recording subject.). Therefore, the present invention will be described referring to the case where the porous film is used for a recording subject; however, it should be noted that the porous film of the present invention is applicable not only to a recording subject, but also to various applications.
2. Prior Art
In the ink-jet recording process, ink is sprayed in the form of droplet onto the surface of a recording subject to do printing thereon. The ink-jet recording method is widely used because of its advantages in that only low noise is produced, the printing speed is high, and clear multicolored-printing can be made. In recent years, as color printers come into wide use, ink-jet printers are increasingly popular to common people.
In the ink-jet printing method, water-based ink is mainly used because it attains high printing performance and is easy on the environment. However, when water-based ink for ink-jet printing is used to do printing on a film for over head projector (OHP), the water-based ink is repelled by the OHP film or spreads on the OHP film, and clear printing is not fixed. This is because an OHP film is made of polyethylene telephthalate (PET), and therefore, has low hydrophilic property.
In order to fix clear printing on an OHP film by an ink-jet printer, the following proposal has been made. A layer made of hydrophilic material is formed on the surface of the PET film so as to receive and fix water-based ink. For example, Japanese Laid-Open Patent Publication No. 57-14091 discloses an invention where a porous layer comprising a binding resin and filling particles such as organic particles and/or colloidal silica having an average particle diameter of 0.1 to 3 .mu.m is formed. However, further studies conducted by the present inventors have revealed that it is difficult to determine the optimum blending ratio between the binding resin and the filling particles. In addition, the obtained porous layer has low transparency. When the amount of resin is excessive, water-based ink cannot be firmly fixed on the layer. Contrary to this, when the amount of resin is too small (that is, the amount of the filling particles is too large), the filling particles may peel off from the surface of the substrate, or the secondary coagulation of the filling particles may occur. As a result, the printed image has poor resolution.
Japanese Laid-Open Patent Publication No. 5-32414 discloses a recording sheet comprising a PET substrate and a porous layer formed thereon. The porous layer is made of pseudo-boehmite including a coagulate of boehmite crystals. The pseudo-boehmite has a particle thickness of 70 .ANG. or larger in a direction vertical to (010) face, and a spacing between (020) faces of 6.17 .ANG. or smaller. Ink is satisfactorily fixed on the sheet, and the recording sheet of this invention has been put on the market. However, this sheet has disadvantages in that fingerprint are easily left thereon, and the color of the sheet easily changes when it adsorbs gas. There are also problems that the sheet has a bad smell which derives from acetic acid used in its production, and that its transparency is relatively low.
The problem of low transparency may be solved by making pseudo-boehmite particle smaller. In this case, however, a fine inorganic particle has too high surface activity to the resultant sheet and coagulates easily. Therefore obtained small pseudo-boehmite particle is hard to handle, accordingly, this approach cannot be adopted. In addition, there is an opinion from general consumers that this sheet is too expensive (the price of this sheet is four or more times the price of standard OHP films).
As a low-priced OHP film for ink-jet printing, a film produced by coating a water-soluble polymer onto a PET substrate is placed on the market. However, this OHP film is poor in humidity resistance, because a water-soluble polymer layer is used as an ink-receiving layer. In order to overcome this disadvantage, this OHP film needs to be packed into a moisture-resistant package; otherwise, blocking may occur, and if moisture adheres to the OHP film after printing, the water-soluble polymer layer dissolves and peels off.